If you look at the guardian at the character creation screen:and then, the guy in T3 armour:

you can see that the T3 set creates a line at the end of his neck. I imagine once that is combined with environmental lightning you could end up with a problem like the one you posted. Could the origin of the problem be that the T3 helmet ends in that place, so A.Net needed to fill that space with a non-armour texture?

In comparison, my face/tattoo combo of choice, also suffers from that issue once I display him in T3 armour: